The present invention relates to light absorbing devices and, in particular, to a reversible thermochromic optical limiter that absorbs laser light while transmitting ambient light.
Optical sensors, including the human eye, can be damaged irreparably by exposure to laser beams. The proliferation of laser devices poses a significant threat to military personnel and electro-optical sensors on the modern battlefield, for example. Clearly, a need exists for materials and methodologies to counter threats from a variety of pulsed and continuous wave laser sources that cover the visible spectrum.
Eye and sensor protection can be provided by absorbers of selected wavelengths of radiation, such as the absorptive goggles presently used for laser eye protection in optical research laboratories. In the military arena, however, the nature of enemy deployed laser devices may be unknown and ever changing. As a result, protection equipment for eyes and sensors must be operative against a broad band of damaging radiation. Because this broad band of potential laser threats includes the entire visible spectrum, conventional absorptive-type protection devices that use dyes to block light are unacceptable. Such conventional broad band absorbers would also block transmission of ambient light during non-threatening periods.
Several technologies are being explored to provide sensor and eye protection from laser light. For example, rugate filter technology has demonstrated the successful deposition of a high transmission filter having six distinct narrow bands of rejection (i.e., lines). The rejection lines of a rugate filter are fixed, however, and must be designed into the structure to counter a known threat. Thus, rugate technology is not adequate for protection against laser beams of unknown wavelength.
Nonlinear optical materials and devices are also being investigated to protect against multiple wavelength and wavelength agile (i.e., rapidly changing) laser threats. For example, Flexible Rejection Filters under development combine rugate filter and holographic technologies for real time sensor projection against pulsed or continuous wave lasers. These filters are made holographically and therefore match the laser wavelengths and directions of arrival. They also operate over a large field of view and a large spectral range.
Another approach for laser eye protection involves thermally triggered sacrificial mirrors. This technology offers rapid response, broad spectral coverage, and high sensitivity against pulsed, continuous wave, wavelength agile, and multiple wavelength laser threats. Sacrificial mirrors, however, provide one-time protection and must be disposed of after illumination by a laser threat.
All of the foregoing technologies have limitations, including excessive cost, that make them poor candidates for widely used laser protection devices. What is needed is a protective device that allows a user or sensor to function normally with ambient light when laser threats are not present. The device should be wavelength agile and exhibit broadband absorption throughout at least the visible region of the spectrum. The device should also be inexpensive and reversible so that it is not destroyed while providing laser protection.
The present invention comprises a reversible thermochromic optical limiter. The device incorporates thermochromic materials that have molecules present in one of two states. At ambient temperatures, the majority of the molecules are transparent to visible light while the remainder of the molecules are present in a colored state. The colored molecules absorb laser radiation, which is converted to heat and deposited in the surrounding nonlinear medium. As the temperature rises, the surrounding non-absorbing molecules transition to the colored state, thereby absorbing more laser radiation, generating more heat, causing further transitions to the absorbing state, and effectively limiting transmission of the laser radiation.
A thermochromic laser protection device of the present invention comprises an optical focusing assembly, a nonlinear thermochromic medium, and a defocusing (i.e., collimating) assembly. A far field image, which may include an intrusive laser beam, is focused through the thermochromic medium. The laser beam is focused to a very small volume, which causes local heating, a large increase in optical density, and absorption of the laser light. The far field image is focused to a much larger volume in the thermochromic medium so that it does not cause significant heating or change in optical density. As a result, the image passes through the device with a dark spot in the far field where the laser originated. The thermochromic medium has a fast response time and returns to its transparent state when the laser beam subsides. The performance of the thermochromic optical limiter depends on the choice of thermochromic material, the medium that carries the thermochromic material, and device parameters such as lens focal length and thickness of the nonlinear element. The selection of a thermochromic material or mixture of materials depends on the response time, dynamic range, spectral range, and recovery time required for a particular application.
A principal object of the invention is a reversible, broad band optical limiter. A feature of the invention is a thermochromic material that absorbs laser light while transmitting an ambient light image. An advantage of the invention is a reversible laser protection device having low cost and fast response time over a broad wavelength band.